With the advent of the adiabatic diesel engine, there has arisen a need for operating an internal combustion engine at higher temperatures without significant loss of heat to an engine cooling system. Ceramics have been suggested for use to implement these ideas. Nowhere is the need for ceramics more important than in the use of an aluminum piston. Several approaches have been used by the prior art to apply ceramics to current production pistons. Mechanical assemblage of a ceramic cap to the piston has been proposed: see U.S. Pat. No. 4,404,935, utilizing a spring biased interlocking shoulder; see U.S. Pat. Nos. 2,257,236, 1,743,323, for securing the ceramic cap to the piston skirt by matching grooves; and see U.S. Pat. No. 1,357,851, for securing such cap by use of threads. Each of these mechanical means may ultimately result in mechanical failure of the ceramic cap due to pressure sensitivity of the brittle ceramic and/or the thermal expansion differential between the ceramic and the supporting metal. In addition, each of these means have their own distinct disadvantage, for example, the spring biased assembly suffers from poor pressure sealing for engine operation.
Ceramics have also been applied to a metallic piston by spray-on techniques (see U.S. Pat. No. 2,833,264), or sintering of the ceramic to the piston metal (see U.S. Pat. No. 2,657,961). These approaches have failed because thick coatings crack due to high thermal gradients and differential thermal expansion, and thin coatings do not provide a sufficient amount of insulation to be worthwhile.
A significantly new approach is to support a preformed ceramic member in a ferrous metal cap or ring which in turn is attached to the aluminum piston body. The ferrous metal cap or ring is sufficiently close in thermal expansion to some ceramics, such as partially stabilized zirconia, to eliminate cracking due to differential thermal expansion. However, there still remains a thermal expansion difference between the ferrous cap and the aluminum piston, which problem must be remedied.
It is not sufficient in solving the latter problem to reverse the use of the materials by making the cap of aluminum and the piston of cast iron. Such reversal of material by various means has been suggested by the prior art. For example, in U.S. Pat. No. 1,771,771, the cap was cast in place with the piston body utilizing double interlocking surfaces which did not totally eliminate rotary looseness; in U.S. Pat. No. 1,388,552, the aluminum cap was tied to the cast iron body by pins, which approach could not realize weight economics and promoted cracking of the ceramic due to the rigidity of the connection. In U.S. Pat. No. 4,364,159, an iron ring was shrunk fit onto an aluminum piston, which ring is incapable of supporting a ceramic insert.
Efforts have been made to cast in place a nonferrous ring with an aluminum piston as disclosed in U.S. Pat. No. 3,152,523, utilizing a titanium cap, and in U.S. Pat. No. 4,334,507, utilizing a nickel-copper or chrome-nickel powder cap. Each of these patents achieves some degree of interlock. In the '507 patent, this was accomplished by filling the pores of the powder with the aluminum melt; this presents a problem because it does not accommodate differential thermal expansion problems. In the '523 patent, a key-shaped interlock was employed; again the problem associated with differential thermal expansion is not addressed, and residual stresses in the aluminum can lead to failure of the piston.
It would be desirable if a method could be devised for securing a ferrous ring (which is capable of supporting a ceramic cap) to an aluminum piston to achieve tight, concentric interengagement between two dissimilar material cylinders at all operating temperatures of the engine and which secure engagement can be achieved economically and with elementary parts.